<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        #sre {
            width: 200px;
            height: 100px;
            border: red 1px solid;
        }
    </style>
</head>

<body>
    <button>dianji</button>
    <div id="sre">

    </div>
</body>

</html>
<script>
    const sre = document.getElementById('sre');
    const btn = document.getElementsByTagName('button')[0];
    btn.addEventListener('click', function() {
        const xhr = new XMLHttpRequest();
        //超时设置
        xhr.timeout = 2000;
        //超时回调
        xhr.ontimeout = function() {
                alert("网络超时");
            }
            //异常回调
        xhr.onerror = function() {
            alert('网络异常');
        }
        xhr.open('GET', 'http://127.0.0.1:8080/srca');
        xhr.send();
        xhr.onreadystatechange = function() {
            if (xhr.readyState === 4) {
                if (xhr.status >= 200 && xhr.status < 300) {
                    sre.innerHTML = xhr.response;
                }
            }
        }
    })
</script>